Abstract:

Although the application of small quantities of nitrogen fertiliser has improved cereal
yields on low-input farms in semi-arid Zimbabwe, the practice is reported to be laborious
and time-consuming by farmers. In an effort to make micro-dosing less labour-intensive
and more precise, an ammonium nitrate (AN) tablet the equivalent of a micro-dose of
prill AN (28 kg N ha-1 ) applied per maize plant was developed by ICRISAT in
collaboration with Agri-seeds, Zimbabwe. This study characterized the physical stability,
chemical (N% and solubility) and agronomic performance of AN tablets compared to
prill. Only 10% of tablets broke when dropped from 2 m showing that they are
physically stable and can handle rough treatment The N content in the tablets (33.3 %)
was comparable to that in prill AN (34.6%). However, the tablet formulation took twice
as long to dissolve than prill AN when placed on a wet soil. Despite this difference in
solubility, simple leaching column experiments suggest that less than 2% of the total AN
applied was lost due to leaching. Agronomic trials were super-imposed on the pairedplot
demonstrations used to promote micro-dosing and the conservation agriculture tillage technique of planting basins from 2005 to 2008. Each tillage (plough and basins)
plot was sub-divided into three sub-plots on which the no AN, prill AN and tableted AN
treatments were super-imposed. Maize was planted and management of plots was left to
the farmer. Micro-dosing with either prill or tableted AN significantly (P < 0.001)
increased maize grain yield by over 40% in all seasons for planting basins. However, on
the ploughed plot there was no yield benefit to using either AN formulation in the season
with the lowest rainfall (2006/07). There was no significant difference in grain yield and
agronomic nitrogen use efficiency between prill and tableted AN formulations except for
2005/06 season in the planting basins. In this season in planting basins, tableted AN
had significantly (P < 0.001) higher rainwater productivity than prill AN which translated
into greater grain yield. In addition, the most benefit to micro-dosing was observed to
accrue when combined with water harvesting techniques such as planting basins. An
observation supported by the host farmers, who in the second and third seasons chose to
apply available basal soil fertility amendments to the basin plots over the flat plots. Thus,
AN tablets if available at an affordable price can be used by smallholder farmers to more
precisely apply N fertiliser. Future work should focus on the labour issues of microdosing
and making cost-effective tablets available to resource-poor farmer, and also
addressing other limiting soil nutrients.